In general, a wireless communication apparatus, such as a radio receiver, has an AGC (Automatic Gain Control) circuit for adjusting the gain of a received signal. The RF (Radio Frequency) AGC circuit is designed to adjust the gain of an RF signal received by an antenna to keep the received signal at a constant level. The RF-AGC circuit will not be activated when the electric field intensity of the signal inputted to the antenna is smaller than or equal to a threshold value and hence will not reduce the gain of the received signal. However, when a signal having an intense field greater than the threshold value is inputted to the antenna, the RF-AGC circuit is activated to reduce the gain of the received signal (see the patent documents 1 and 2, for example).    Patent Document 1: Japanese Patent Laid-Open No. 6-276116    Patent Document 2: Japanese Patent Laid-Open No. 2004-48177
Such an AGC circuit is effective in reducing the level of interference waves contained in the received signal. For example, when a strong interference wave is inputted (and hence the received field intensity exceeds the threshold value), reducing the gain of the RF signal can reduce the signal level of the interference wave. However, the RF signal, the gain of which is to be reduced, contains a signal at a desired frequency as well as the interference wave noise signal. Therefore, activation of the RF-AGC circuit disadvantageously reduces the level of the signal at the desired frequency at the same time.
Thus, for example, when the received field intensity exceeds the threshold value as a result of reception of the strong interference wave even though the level of the signal at the desired frequency is small, the AGC circuit reduces the gain of the RF signal, resulting in further reduction of the level of the signal at the desired frequency, which is small in the first place. Reception sensitivity for the desired channel is therefore significantly reduced, resulting in a disadvantage of almost inaudible audio of interest.
To avoid such inconvenience, there has been proposed a technology in which the field intensity at a desired frequency is detected, and the AGC sensitivity is reduced when the detected field intensity is smaller than a threshold value, so that the AGC operation is not activated when the field intensity at the desired channel is weak (see the non-patent document 1, for example).
Non-patent Document 1: Development of High-performance FM Front End IC with improved interference characteristic; IEEE Transactions on Consumer Electronics, Vol. 37, No. 3, AUGUST 1991 by Sanyo Electric Co., Ltd.
FIG. 1 shows the characteristics of an AGC operation achieved by the technology described in the non-patent document 1. As shown in FIG. 1, according to the non-patent document 1, in a medium/intense field area where the detected voltage Vcont corresponding to the field intensity at a desired frequency is greater than or equal to 2 [V], the AGC operation is turned on when the field intensity of the signal inputted to the antenna exceeds 60 [dBμ]. On the other hand, in a weak field area where the detected voltage Vcont is smaller than 2 [V], the threshold value at which the AGC operation is turned on is increased to 85 [dBμ], so that the AGC operation will not immediately be turned on even if the field intensity of the signal inputted to the antenna exceeds 60 [dBμ].
In the technology described in the non-patent document 1, however, the operation described above is achieved with an IC having an analog circuit configuration. The threshold value that defines the boundary between the medium/intense field area and the weak field area (hereinafter referred to as “weak field start level”) is a fixed value, and this value cannot be changed once the IC has been designed. The threshold value that defines the boundary at which it is judged whether or not the AGC operation is activated in the weak field area (hereinafter referred to as “AGC start level”) is also a fixed value, and this value cannot be changed once the IC has been designed.
In general, designers of ICs including AGC circuits do not know what products the ICs are applied to or what situations the ICs are used in. It is therefore difficult for the designers to determine a optimum value of the weak field start level and the AGC start level. In fact, conditions including reception of desired waves and the way interference waves are received are totally different depending on the usage of the IC, that is, whether the IC is used in a radio receiver, in a mobile phone, or in other wireless communication apparatuses. Even when the IC is used in products of the same category, for example, radio receivers, the reception of desired waves and interference waves is totally different depending on whether the IC is incorporated in a home-use audio apparatus or a car radio receiver. Thus, every product in which an IC including an AGC circuit is used has its own weak field start level and AGC start level required to perform optimum RF-AGC control without compromising reception sensitivity of a desired wave in a weak field area.
That is, it is desired in the first place that a manufacturer or the like of a product in which an IC is incorporated carries out a field test or the like and resultant optimum values are used as the weak field start level and the AGC start level. As described above, however, in the technology described in the non-patent document 1, the weak field start level and the AGC start level are fixed values, so that they cannot be customized. Optimum RF-AGC control without compromising reception sensitivity of desired waves in a weak field area is therefore practically not possible.